The present invention generally relates to measuring the bandwidth of optical multimode and few-mode fibers (MMFs and FMFs) and, more particularly, relates to methods and configurations for testing MMFs and FMFs by light transmission through the fiber.
Bandwidth is an important attribute of MMF and FMF products. Ultimately, bandwidth is used in the optical fiber industry to set the grade of the MMF and FMF products. The bandwidth is directly related to how the refractive index profile of the MMF and FMF exists in practice compared to the ideal, optimal profile for a given fiber configuration. The bandwidth exhibited by a given MMF or FMF is generally very sensitive to even a slight deviation from the ideal, optimal profile.
One method for measuring the bandwidth of an MMF or FMF is to apply a light signal at one end of the fiber, receive the transmitted light signal at the other end, and measure bandwidth through the known, differential mode delay (DMD) measurement technique. The DMD method records the time delay of the transmitted signal through the fiber. The method involves launching the light signal from a single mode test fiber, offset from the center-line of the MMF or FMF on test, and this offset is varied across the face of the MMF or FMF under test. Further, data processing is used to convert the time pulses versus radial offset position into a bandwidth result.
Usually, a short pulsed laser is employed to transmit the light signal through the MMF or FMF. The short pulsed laser in these test configurations is typically limited to one wavelength, around 850 nm. Ti-sapphire lasers can be employed in these configurations, but are expensive and difficult to maintain.